Compensation system for a magnetic compass

ABSTRACT

In a vehicle locating apparatus that utilizes Hall effect generators to determine the magnetic heading of the vehicle there is provided magnetic compensation by the placing of a coil around the Hall generators. A current source generator provides power to the coil of such magnitude so as to create a magnetic field which cancels the spurious local magnetic field components. The system includes a set of manual switches for inputting a rotation angle. A variable time delay circuit incorporates this rotation angle to electronically increase or decrease the measured magnetic heading angle. This electronic rotation provides for a method to correct physical misalignment of the heading sensor in the vehicle and to compensate for local magnetic deviation.

FIELD OF THE INVENTION

The present invention relates to vehicle locating apparatus and moreparticularly to a vehicle locating apparatus utilizing an automaticallyupdated dead reckoning system.

BACKGROUND OF THE INVENTION

A number of systems have been proposed for performing a vehicle locatingfunction by utilizing an onboard compass and distance measuring devicewith dead reckoning and automatic position updating. The updating occurswhenever the vehicle passes a signpost that is transmitting itslocation. These systems are generally disclosed in U.S. Pat. No.3,749,893 and 3,961,166. In actual operation such systems encounter anumber of problems. These problems include compass errors caused by themetallic body of the vehicle, permanently magnetized iron in theproximity of the heading sensor and deviations in the earth's magneticfield. Other problems which have been encountered include inaccuraciesdue to varying wheel diameters, inconvenient map scale units, increasedexpense due to the high accuracy oscillators required in the updatetransmitters and receivers, and excessive time required to physicallyalign the heading sensor in the vehicle.

Thus, there is a need for improvements over the existing vehiclelocating systems using dead reckoning with proximity signpost updating.

SUMMARY OF THE INVENTION

A vehicle location system utilizing Hall effect generators to determinemagnetic heading is provided with a compensation system that has anelectrically conductive coil encircling the Hall effect generator andcarrying current of such magnitude to cancel local magnetic fieldcomponents, that is, except the earth's horizontal field.

Further, the vehicle locating apparatus of the present inventionutilizes wheel rotation to measure distance traveled and provides forprogramming a constant number into the locating system to compensate forvarying wheel circumferences and to incorporate selectable map scaleunits. This number is the quotient of the map scale unit divided by thewheel circumference and is repeatedly added to or subtracted from anaccumulator that is measuring distance traveled. The number is added orsubtracted to maintain the accumulator within a narrow range of values.The number of additions or subtractions is counted and stored andrepresents the distance traveled as measured by the selected map scaleunit.

The vehicle locating system of the present invention also utilizes anupdate signpost transmitter in which the data format provides aself-clocking feature. The basic data rate is subdivided into threesubperiods with the first and last of the subperiods at fixed butdifferent logic levels. The received demodulation logic detects thetransition from the third subperiod to the first subperiod and generatesa sampling pulse that occurs in the middle of the second subperiod. Thesecond subperiod is data dependent and the generated pulse samples theincoming data to determine its logic level.

The vehicle locating system is provided with means for manuallyprogramming electronic rotation of the heading angle. This meansincludes a set of switch inputs that operate a variable time delaycircuit that delays the heading sensor output relative to the sensorinput to provide the desired rotation.

BRIEF DESCRIPTION OF THE DRAWINGS

The novel features characteristic of the invention are set forth in theappended claims. The invention itself, however, as well as furtheradvantages thereof, will best be understood by reference to thefollowing detailed description of an illustrative embodiment taken inconjunction with the accompanying drawings, in which:

FIG. 1 is a block diagram of the vehicle locating system;

FIG. 2 is a schematic diagram of the Hall effect sensor compensationcircuit;

FIG. 3 is a schematic diagram for the map scale factor and wheeldiameter compensation circuit;

FIG. 4 is a block diagram of the map scale factor and wheel diametercompensation circuit;

FIG. 5 is a logic diagram for the update transmitter modulator;

FIG. 6 in a waveform chart for the logic diagram of FIG. 5;

FIG. 7 is a logic diagram for the update data demodulator;

FIG. 8 is a waveform chart for the logic diagram of FIG. 7;

FIG. 9 is a block diagram of the electronic rotation circuitry;

FIG. 10 is a waveform chart for the rotation circuitry of FIG. 9.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The functional operation of the vehicle locating system is shown in theblock diagram of FIG. 1. The Hall effect heading sensor 3 determines themagnetic heading A of the vehicle and is driven by the Hall effectgenerator driver 7. Transverse and longitudinal compensation areprovided by drivers 2 and 5 to the heading sensor 3. Compensationmagnitude and polarity for both the transverse and longitudinal elementsare provided by selection switches 1 and 4. The output of Hall effectheading sensor 3 is the analog value of the heading angle A which isdigitized in encoder 8. Cosine generator 9 produces the cosine of theheading angle A which is input to the location computation logic 10. Thecurrent vehicle location is stored in register 27 and this location istransmitted to the central receiving station 40 by the vehicle locatingtransmitter 28.

The analog value of the heading angle A is rotated by an amount selectedby switches 6. Rotation of the vehicle wheels is detected by sensor 19and input to the location computation logic 10. K factor selectionswitches 20 input to the location computation logic 10 a value tocompensate for varying wheel diameters and to determine the scale factorin which the vehicle's location is reported.

A signpost position unit 50 consists of a position generator 47 whichoutputs serial update data to a signpost update logic modulator 48. Thisdata is then transmitted by the signpost transmitter 49 to the vehicleupdate position receiver 29. The received data is conveyed to thesignpost update logic demodulator 30 and then to the vehicle locationstorage registers 27.

The operation of the vehicle location system is described in referenceto FIG. 1. The Hall effect heading sensor 3 determines true magneticheading due to the compensation provided by drivers 2 and 5. Thiscompensation minimizes the local magnetic components except the earth'snatural horizontal magnetic field. The magnitude and polarity of thecompensation is determined by inputs to the selection switches 1 and 4and the cosine of the measured heading angle A.

The electronic rotation selection switches 6 provide a method forcompensation for physical misalignment of the heading sensor 3 withinthe vehicle and for the local magnetic deviation. The numerical valueselected in the electronic rotation selection switches 6 either advanceor retard the heading angle A provided by the encoder 8.

The K factor selection switches input a number between the values of 1and 63 and 63/64ths. This factor is utilized by the location computationlogic 10 to accommodate the distance measurements provided by wheelrotation sensor 19 and to provide a position output in terms of apredetermined map scale factor.

The vehicle location storage registers contain the current vehiclelocation as determined by the location computation logic. But when thevehicle passes within the proximity of a signpost update unit 50 anupdated vehicle position is input to the vehicle location storageregisters 27.

The signpost position generator 47 repetitively generates the serialupdate position of the signpost. The signpost updata data logicmodulator 48 modulates the update data in a format which isself-clocking and reduces the need for accurate oscillators. This datais transferred by the signpost transmitter 49 to the vehicle updateposition receiver 29. The output of receiver 29 is demodulated in thesignpost update data logic demodulator 30 which then provides thesignpost location to the vehicle location storage registers 27 andgenerates the clocking function necessary for demodulating the incomingdata.

Referring to FIG. 2, the positive or negative cosine of a heading angleA is selectively connected by two position switch 11 to the negativeinput of an amplifier 15 through a variable resistor 12 and a seriesinput resistor 14. Also included in the input circuit of the amplifier15 is a capacitor 13 tied at one terminal to ground. A positive ornegative DC voltage is selectively connected by a two position switch 21to the negative input of the amplifier 15 through a variable resistor 22and a series input resistor 24. Also forming a part of this inputcircuit is a capacitor 23 tied at one terminal to ground.

The freely swinging member of a pendulous resistor 41 is connectedthrough an input resistor 42 to the negative input of an amplifier 44.The end terminals of the resistive body of the pendulous resistor 41 aretied to positive and negative DC voltages. Connected to the positiveinput of the amplifier 44 is an input resistor 43 with a grounded secondterminal. In the feedback loop of the amplifier 44, connected betweenits output and its negative input, is a resistor 35 in parallel with theseries combination of a capacitor 33 and a capacitor 34. The output ofamplifier 44 is connected through the series combination of a resistor32, a variable resistor 31 and a resistor 39 also to the negative inputof the amplifier 15. Also included as a part of this input circuit ofthe amplifier 15 is a capacitor 38 having one terminal tied to ground.Connected to the positive input of the amplifier 15 is an input resistor16 with a grounded second terminal.

Forming the feedback loop of the amplifier 15, connected between theoutput and its negative input is a feedback capacitor 17. Also tied tothe output of the amplifier 15 is an input resistor 26 connected to thebase terminals of power transistors 36 and 37. These transistorsconstitute the driving amplifier for a transverse coil 46 through aninput resistor 45. The output voltage at the emitter electrodes oftransistors 36 and 37 is connected through a feedback resistor 25 to thenegative input of the amplifier 15.

The positive or negative cosine of the heading angle A is alsoselectively connected by a two position switch 54 to the negative inputof an amplifier 75 through a variable resistor 51 and a series inputresistor 53. Also included as a part of this input circuit to theamplifier 75 is a capacitor 52 that is tied at one terminal to ground. Apositive or negative DC voltage is also selectively connected by a twoposition switch 68 to the negative input of the amplifier 75 through avariable resistor 55 and a series input resistor 57. This input circuitto the amplifier 75 further includes a capacitor 56 that is tied at oneterminal to ground.

A pendulous resistor 65 has a pendulum arm that is free to swing in thelongitudinal axis of the vehicle, and has a resistive element connectedto positive and negative DC voltages. The pendulous element is connectedthrough series input resistor 66 to the negative input of an amplifier73. A series resistor 72 is connected at one terminal to the positiveinput of the amplifier 73 and to ground at the other terminal. Feedbackelements for the amplifier 73 are connected between its output and itsnegative input and consists of a resistor 71 in parallel with acapacitor 67.

The output of the amplifier 73 is connected through the seriescombination of a resistor 62, a variable resistor 61 and a resistor 64to the negative input of the amplifier 75. A capacitor 63 is tied at oneterminal to the negative input circuit of the amplifier 75 and at theother terminal to ground. A resistor 74 is connected at one terminal tothe positive input to the amplifier 75 and at the other terminal toground.

The output signal of the amplifier 75 is connected through the seriesresistor 79 to the base electrodes of driver transistors 81 and 82. Thisoutput signal is also connected through feedback capacitor 76 to thenegative input of the amplifier 75. The transistors 81 and 82 withinterconnected emitters constitute an amplifier which drives a headingcoil 84 through a load resistor 83. The output of the driver amplifier,consisting of transistors 81 and 82, is connected through feedbackresistor 77 to the negative input of the amplifier 75.

The operation of the compensation circuit can be more fully understoodby referring to FIG. 2. The detector portion of the solid state compassconsists of two Hall effect generators orthogonally mounted in thehorizontal plane. Approximately 200 turns of wire are wrapped aroundeach of the Hall effect generators. These coils are shown as transversecoil 46 and heading coil 84. Current is caused to flow through these twocoils in such a manner as to create a magnetic field which is equal inmagnitude but opposite in direction to the unwanted magnetic fieldsaffecting the generators. These induced magnetic fields cancel theunwanted magnetic fields, thereby leaving the detector to measure onlythe earth's natural horizontal magnetic field. The unwanted magneticfields are of three types. Fundamental interference that is caused bythe presence of any permanently magnetized body on the vehicle itself.Second harmonic interference that is caused by the presence of any softiron near the magnetic detector. And, the third type of interferenceconsists of unwanted vertical components of the earth's magnetic fieldthat are introduced into the detector by either roll or pitch of thedetector.

The transverse coil 46 provides compensation for fundamentalinterference, second harmonic interference, and roll interference. Thecompensation current for fundamental interference is a constant valuewhich is determined by variable resistor 22. The compensation currentfor the second harmonic interference is a function of the heading angleA the cosine of which is supplied through switch 11. The output ofswitch 11 is provided to variable resistor 12 which is adjusted todetermine the magnitude of the compensation current for second harmonicinterference. The roll compensation current is determined by thependulous variable resistor 41. This pendulum element is free to swingonly in the transverse plane. The output of variable resistor 41 is fedthrough amplifier 44 and its associated circuitry then through resistor32 and variable resistor 31 which determines the magnitude of the rollcompensation. These three compensation currents, fundamental, secondharmonic and roll, are summed at point 18. These summed compensationcurrents are then amplified by amplifier 15 and then applied to thecombination of transistors 36 and 37 with the resulting current directedto the compensation coil 46. The current in coil 46 then creates theappropriate counteracting magnetic field to cancel the spurious magneticfield components.

In a similar fashion compensation currents for fundametal interference,second harmonic interference and pitch are generated and fed to summingpoint 78. The pendulum element of variable resistor 65 is allowed toswing only in the longitudinal plane. The resulting current is thenamplified and fed to heading coil 84 to create the appropriatecounteracting magnetic field to cancel the spurious magnetic fieldcomponents associated with the heading coil.

Thus, in this manner, the Hall effect generators will respond only tothe horizontal component of the earth's natural magnetic field andthereby produce an accurate magnetic heading.

Referring now to FIG. 3, a map scale factor and wheel diameteradjustment constant K is programmed into the system with a bank of inputswitches 101-112. Each of these switches is connected to the positivevoltage supply. The contact arm of the switches are connected throughresistors 121-132, respectively, to ground. Thus, when a switch is openthe line going to the logic is at a low level, but when the switch isclosed the line voltage goes to a high level. The input switches 105-112are connected to a shift register 140 and input switches 101-104 areconnected to shift registers 141 and 142. Connected to the output ofshift register 142 by means of a line 145 is the input of an inverter144. A flip-flop 143 is connected to registers 140, 141 and 142.

A clock signal is input on line 152 and a bit timing signal is input online 151.

In the present embodiment the shift register 140 is an integratedcircuit type CD 4014 chip, registers 141 and 142 are integrated circuitstype CD 4035, and flip-flop 143 is a type CD 4027 chip. The pinconnections are as shown in FIG. 3.

Operationally, the constant K is programmed into the system in powers of2 as shown in parentheses beside the corresponding switch. The value ofthe constant K in this embodiment ranges from a minimum of 1 to amaximum of 63 and 63/64ths. Each of the switches 101-112 corresponds toa power of 2 ranging from minus 6 to positive 5. For example, if theconstant K is 2 3/16, the switches 108, 104, and 103 would be closedwith the remaining switches left open. The logic levels of switches105-112 are shifted in parallel fashion into shift register 140. Thelogic levels of switches 101-104 are shifted in parallel fashion intoshift registers 141 and 142. The value of K is converted into serialform and output on line 145. The output of inverter 144 on line 153 inthe 1's complement of K, as represented by K. The 2's complement of K,represented as -K, is produced on line 146.

The functional operation of the value K in the vehicle locating systemis shown in FIG. 4. A heading sensor 161 determines the magnetic headingangle A of the vehicle. The analog value of the heading angle A istransferred into an encoder 162 where it is digitized and the digitalheading is then applied to a trigonometric function circuit 166 whichproduces the cosine of the heading angle. The cosine of the headingangle is then transmitted through switch 163 to the upper input of adigital summer 164 with the output of the digital summer 164 going to anaccumulating register 165. One output of the accumulating register 165goes to a multiply-by-two circuit 171. The output of multiply-by-twocircuit 171 is transferred to one input of digital summers 172 and 175.The output of summer 172 is conveyed through an inverter 174 to the inutof a D-type flip-flop 173. The output of the summer 175 is transferredto the input of a D-type flip-flop 176. The outputs of the flip-flops173 and 176 are selectively connected to a register 181.

The two's complement of the number K is input on line 182. The one'scomplement of the number K is input on line 183 and the numerical valueof K is input on line 185. Bit timing for the flip-flops 173 and 176 isinput on line 184 from a bit time clock source.

In the present embodiment, digital summers 164, 172 and 175 and inverter174 are in a type CD 4032 AE integrated circuit. The flip-flops 173 and176 are in a type CD 4013 integrated circuit.

The value K determines the map unit scale factor that is used to reportthe movement of the vehicle. It is the quotient of the map scale unitdivided by the wheel circumference. The value of K can also becalculated as follows. The vehicle is driven a distance of at least 10miles where the distance can be measured accurately to within 1/2%.During the drive, the number of wheel rotations is counted. Next, anarbitrary scale factor is selected, for example 10 feet or 10 meters. Kis then calculated by dividing the number of wheel revolutions by thequotient of the distance traveled divided by the scale factor. Thefractional portion of the value K must then be converted to a fractionin 64ths. For example, if the distance traveled is 11 miles and thenumber of wheel revolutions is 8249 and the scale factor is 10 feet theresulting value of K is 1 and 27/64ths. Thus, the tire diameter has beenincorporated into the system and the movement of the vehicle will bereported in units of 10s of feet.

Referring to FIG. 4, for each wheel rotation the value of the cosine ofthe heading angle A is added into accumulating register 165.Accumulating register 165 acts as an accumulator storing each value ofthe cosine of the heading with the distance value of each unit inaccumulating register 165 equal to one scaled wheel circumference. Thecombination of the summer 172, the inverter 174 and the flip-flop 173performs a monitoring function by providing an output on line 177 whenthe value stored in register 165 is greater than one-half the value ofK. A monitoring function is also provided by the summer 175 and theflip-flop 176 as an output on line 178 when the value stored in register165 becomes less than one-half the negative value of K.

Register 181 performs an accumulator function for the K overflow valueson lines 177 and 178 and indicates the north/south location of thevehicle in the units that were chosen to determine the value of K. Thenumber stored in register 181 is the distance the vehicle travels from apredetermined reference point. This is the value of Y in a conventionalX-Y Cartesian coordinate system. When there is an output on line 177 oneunit is added to register 181. When there is an output on line 178 oneunit is subtracted from register 181.

When any change is made in register 181 the value stored in register 165is modified by addition of a plus or minus value of K as selected byswitch 163. A minus K value is selected by switch 163 if a positive unitwas added to register 181 and a positive value of K is selected byswitch 163 if a negative unit was added to register 181. This new value,which consists of the old value in register 165 and either plus or minusK, is accumulated in register 165. The overall result of this operationis the dividing of the accumulated heading value by the constant K andthe division is the vehicle location which is stored in register 181 inthe map scale unit selected in determining the value of K.

The process of adding or subtracting the value K to register 165maintains the value in this register within the range of plus or minusK. This technique maintains the maximum possible accuracy with a givensize of register.

Register 181 provides the north/south location of the vehicle from acentral reference point. If in unit 166 the sine of the heading angle Ais used in place of the cosine of the heading angle A then register 181will accumulate the vehicle position in east/west coordinates using theunit selected to determine the value of K.

The update data modulation digital circuitry and signal waveforms forthis circuit are shown in FIGS. 5 and 6, respectively. An integratedcircuit 211 is an oscillator with the rate controlled by two externalcomponents, a variable resistor 221 and a capacitor 222. The output fromthe integrated circuit oscillator 211 is transmitted on line 231 to adivide-by-two integrated circuit 212. The output of the divide-by-twocircuit 212 is transferred by means of line 232 to the clock inputs ofD-type flip-flops 213 and 214. The complement output of the flip-flop213 is conveyed by means of line 233 to the second input of NAND gate216. The second output of the flip-flop 213 is connected through line238 to the input of the flip-flop 214 and to one input of the NAND gate215. The output of the flip-flop 214 is transferred by means of line 234to one input of a NAND gate 215.

The serial data is input on line 235 to the second input of a NAND gate216. The output of the NAND gate 216 is transferred by means of a line214 to one input of the NAND gate 217 that has a second input tied tothe output of the NAND gate 215 by means of a line 236. The output ofthe NAND gate 217 is the logically modulated serial data which isconveyed through line 237 to the modulation amplifier of the updatetransmitter.

In the present embodiment the integrated circuit oscillator 211 is atype CD 4047 chip. The divide-by-two circuit 212 is a type CD 4020 chipand the flip-flops 213 and 214 are combined in a type CD 4013 chip.

FIG. 6 shows the data waveforms for one data period for lines 236, 233,234, 235 (data logic one), 235 (data logic zero), 237 (modulated logicone) and 237 (modulated logic zero) for the circuit of FIG. 4. Data isinput at a rate F with a period 1/F.

Circuitry of FIG. 5 operates to provide the modulation operation of thelogical data transmission technique. The transmitter clock is providedby the oscillator 211. In one embodiment, the output of oscillator 211is an 18 KHz signal transmitted on line 231 to the divide-by-two circuit212. Divide-by-two circuit 212 provides a division-by-two with theoutput being a 9 KHz signal on line 232. Signals 232 on the line aretransmitted to the D-type flip-flops 213 and 214 and applied to the NANDgate 215 where it is divided by three to produce the various signalwaveforms shown as 236, 233 and 234 in FIG. 5. Each of these waveformsis at a 3 KHz rate with a period of one third millisecond. The one thirdmillisecond period is further subdivided into three one ninthmillisecond periods.

As illustrated in FIG. 6, the signal on line 236 is a logic zero for thefirst one third of the basic period and a logic one for the remainingtwo portions of the period. The signal on line 233 is a logic zero forthe first portion, a logic one for the mid portion and a logic zero forthe final portion. The signal on line 234 is a logic one during thefirst two portions and a logic zero during the final third portion. Thisdata transmission system makes the overall vehicle locating system lessdependent upon accurate oscillators such as a crystal oscillator andthis system provides a self-clocking feature.

The serial data consisting of logical ones and zeros is entered on line235. This data is either a logical one or zero for a full bit period,that is, one third of a millisecond. The signal 236 forces the output ofthe NAND gate 217 to always be a logic one during the first one third ofthe bit period. The final third of the bit period on line 237 is alwaysa logic zero because line 233 is a logic zero during this one thirdforcing the output of the NAND gate 216 to a logic one. The other inputto the NAND gate 217 is a logic one during this final one third of theperiod, therefore the output of this gate on line 237 must always be alogic zero during the final third of the bit period. The middle onethird of the bit period is data dependent, that is, upon the input toline 235. During the mid portion of a bit period both lines 233 and 236are logic ones. Thus, if the data on line 235 is a logic one the outputof NAND gate 216 is a logic zero. Then the output of NAND gate 217 willbe a logic one. If the data on line 235 is a logic zero the output ofNAND gate 216 will be a logic one and the resulting output of NAND gate217 will be a logic zero. Thus, the mid portion of the bit period online 237 corresponds to the respective one or zero level of the serialdata input on line 235. The corresponding logical ones and zeros forlines 235 and 237 are also shown in FIG. 6.

The update data demodulation logic is shown in FIG. 7, with the relateddata demodulation waveforms shown in FIG. 8. The line numbers in FIG. 8correspond to the waveforms in FIG. 7. Data is input on line 341 toD-type flip-flops 311 and 332. A 108 KHz clock signal is input on line342 and supplied to the clock inputs of flip-flops 311, 312, 323 and324. The output of the flip-flop 311 is transferred by means of line 343to the input of the flip-flop 312 and to one input of a NAND gate 313.The complement output of flip-flop 312 is transmitted on line 344 to thesecond input of the NAND gate 313. The output of the NAND gate 313 istransmitted by means of line 345 to the clock input of a monostablemultivibrator 314. The monostable multivibrator 314 provides an outputwhose duration is a function of the time constant of an RC networkconsisting of a resistor 315, a variable resistor 322 and a capacitor321. The complement strobed output of the monostable multivibrator 314is transmitted on line 346 to the input of the flip-flop 323. The outputof flip-flop 323 is transmitted over line 347 to the input of theflip-flop 324 and one input of a NAND gate 325. The complement output ofthe D-type flip-flop 324 is transmitted by means of line 348 to thesecond input of the NAND gate 325. The output of the NAND gate 325 istransmitted over line 349 to an inverter 331 and through line 350 to theclock input of the D-type flip-flop 332. The output of flip-flop 332 online 351 is the demodulated update logic data.

In the present embodiment the flip-flops 311, 312, 323, 324 and 332 aretype CD 4013 chips. The monostable multivibrator circuit 314 is a typeCD 4047 chip.

The functional operation of the demodulation logic is described byreference to FIGS. 7 and 8. Data input on line 341 is in the formdescribed in FIG. 5 as the one and zero waveforms 237 in FIG. 6. Eachmodulated logic bit consists of three equal sections the first sectionbeing always a logic one, the center section determining whether the bitis a one or zero and the third section always being a logic zero. Thepurpose of the demodulation logic is to derive a clock pulse train fromthe input data and to determine the one or zero state of the centerone-third section of the bit period.

A 108 KHz clock is provided on line 342 from an oscillator. The inputdata rate is approximately 3000 bits per second. Therefore there will beapproximately thirty-six clock pulses during each data bit. Theflip-flop 311 provides a one clock bit delay to the incoming modulateddata shown as waveform 343. Flip-flop 312 provides another one bit clockdelay and inverts the data as shown in waveform 344. NAND gate 313compares the outputs of the two flip-flops 311 and 312 to detect apositive transition and provides an inverted one clock bit pulse at thetime of this transition. This is shown as waveform 345.

Monostable multivibrator 314 provides an output which lasts for a fixedtime delay after the pulse is received from NAND gate 313. This is shownas waveform 346. Flip-flop 323 provides a one clock bit delay which isshown as waveform 347. Flip-flop 324 inverts the signal and adds anotherone clock bit delay. This is shown as waveform 348. The NAND gate 325compares the outputs of flip-flops 323 and 324 and provides a zerooutput at the time when the input waveforms are both logically one asshown by waveform 349.

The output of NAND gate 325 through the inverter 331 is the update clockwhich is a pulse occurring at the start of each bit of the demodulateddata. Modulated data is input to flip-flop 332 on the line 341. Theupdate clock provides the clock function for flip-flop 332 to strobe theinput data at the center of each received data bit time.

The time constant of resistors 315, 322 and capacitor 321 are set suchthat the update clock occurs after a one-half data period delay,approximately 0.17 milliseconds. Therefore, flip-flop 332 is clocked onat the center of the middle one-third period of the three elementmodulated data period. This provides the sampling function whichdetermines whether the center one-third of a data period is high or lowand the bit is therefore a one or zero. The demodulated data is outputon line 351. This data has a one-half period delay from the inputmodulated data.

The purpose of this data transmission technique with three elementswhere the first is always high, the last is always low and the centerdetermines the one or zero status, is to reduce the oscillatoraccuracies needed in the transmitting and receiving systems. The designis such that the data sample will be taken in the center of the middlehigh or low segment. But the sample can be taken at any time during thismiddle segment. Thus the total drift of the oscillators can be as muchas ± 1/6 of the data rate frequency.

The vehicle locating system is provided with a capability forelectronically rotating the heading angle. This circuitry is shown inFIG. 9 where the switches 421 through 428 provide the numerical inputfor the value of electronic rotation. Resistors 431 through 438 areconnected to the respective switches to provide a low logic level when aswitch is open and a high logic level when the switch is closed. Theoutputs of switches 421 through 428 are connected to a variable delaycircuit 441. The system 1.08 MHz clock is input on line 450 to a dividercircuit 443. The divider circuit 443 provides a divide-by-360 function.The system clock is also provided to circuits 441, 442 and 447. Theoutput of the divider circuit 443 on line 451 is provided to anamplifier 444 and a pulse generator 445. The output of the amplifier 444on line 453 is applied to a heading sensor 446, consisting of two Hallgenerators, and the output of the pulse generator 445 is transmitted bymeans of a line 452 to the fixed delay circuit 447. The output of thefixed delay circuit 447 is provided on line 455 to the input of a phasedifferential measurement circuit 442. The output from the heading sensor446 is provided over a line 454 to a preamplifier 448. The preamplifier448 output on line 449 is one input to the variable delay circuit 441that produces a delayed output on line 456 to the phase differentialmeasurement circuit 442.

The operation of the electronic rotation function is shown in FIGS. 9and 10. The system clock operates at 1.08 MHz and is input on line 450to the divider circuit 443 that provides a divide-by-360 operation toproduce a 3 KHz sine wave on line 451 as shown in FIG. 10. This waveformis input to amplifier 444 which provides the reference drive signal tothe transverse Hall generator. A cosine reference drive signal (notshown) is provided to the heading Hall generator. The generator outputsare summed to produce the heading sensor 446 output. The output of theheading sensor on line 454 is a sine wave with a phase delay equal tothe heading angle. The output of the heading sensor is amplified bypreamplifier 448 to produce a squarewave whose phase is also a functionof the heading angle A.

The numerical value of the electronic delay of circuit 441 is set intothe system by closing the appropriate switches 421 through 428. Thephase delay values in degrees for each switch are shown in parentheses.These switches are connected to variable delay circuit 441 which delaysthe squarewave output signal of the heading sensor preamplifier 448 bythe amount so programmed. This delayed heading sensor output is one ofthe inputs to the phase differential measurement circuit 442.

The signal on line 451 is also input to the pulse generator circuit 445that produces a 3 KHz pulse rate on line 452 as shown in FIG. 9. Delaycircuit 447 provides a fixed delay for the 3 KHz input signal. The phasedelay provided in this embodiment is approximately 60°. This delayed 3KHz pulse signal is input to circuit 442.

Circuit 442 measures the phase differential between the positivetransitions on line 456 and the pulses on line 455. The output, phasedifference, of circuit 442 on line 460 is the measurement of thecompensated heading angle A.

In operation if no heading compensation is desired the switches 421through 428 must be set with an angle of sixty degrees. This will delaythe output of the heading sensor 446 the same amount as the fixed delayof the timing signal on line 455. Thus there will be no timedifferential between the signals on lines 456 and 455 when the sensor ispointed to magnetic north. If it is desired to electronically rotate theheading sensor output to produce a signal that is equivalent to aphysical rotation of ten degrees counterclockwise, the switches 421through 428 must be set to a value of 50°. Under these conditions theheading sensor output signal on line 456 will lead the timing signal online 455 by ten degrees if the heading sensor is facing magnetic north.If it is desired to simulate a physical clockwise rotation of 10° of theheading sensor the switches 421 through 428 must be set to a value of70°. Again if the heading sensor is facing magnetic north this producesa differential of 10 degrees between the signals on lines 455 and 456with the heading sensor signal being the delayed signal. Thus if theheading sensor is improperly physically mounted in the vehicle and isnot facing magnetic north, the compensation can be done electronicallyrather than physically. In addition this circuitry can program tocompensate for the local magnetic deviation in the earth's field.

In FIG. 10 the waveform 451 is the input to amplifier 444 and pulsegenerator 445. Waveform 452 is the output of pulse generator 445. Thepulse is generated at the positive zero transition of the input signal.Waveform 451 is also one of the inputs to heading sensor 446. Waveform449 is a sample output of heading sensor 446 for a heading angle A of 45degrees. Waveform 455 is the output of delay circuit 447. Waveform 456is an example of the output of variable delay circuit 441 for a headingangle A of 45 degrees with the variable delay switches set to 60degrees.

While embodiments of the invention have been described in detail,modifications and alterations may occur to others upon a reading andunderstanding of the specification. It is intended to include all suchmodifications and alterations as fall within the scope of the appendedclaims.

What is claimed is:
 1. A compensation system for a magnetic compassutilizing a Hall effect generator comprising:(a) a compensation coilencircling said generator, and (b) a compensation driver for generatinga current in said coil to create a magnetic field surrounding said coilhaving a magnitude equal to but opposite in direction of local magneticfield components other than the earth's horizontal magnetic field.
 2. Acompensator system for a magnetic compass utilizing a Hall effectgenerator as recited in claim 1 wherein said compensation driverincludes:(a) means for generating a component of the current in saidcoil at a level determined by the presence of magnetized bodies and softiron in the proximity of said compass, and (b) means for generating acomponent of the current in said coil varying with the verticalcomponents of the earth's field introduced into said Hall effectgenerator by the tilting thereof from the horizontal.
 3. A compensationsystem for a magnetic compass utilizing a Hall effect generator asrecited in claim 1 wherein said compensation driver includes meansresponsive to a heading angle signal for generating a current relatedthereto in said coil.
 4. A compensation system for a magnetic compassutilizing a Hall effect generator as recited in claim 1 wherein saidcompensation driver includes a pendulous resistor for generating acomponent of current in said coil related to the pendulous movementthereof.
 5. A compensation system for a magnetic compass utilizing aHall effect generator as recited in claim 1 wherein said compensationdriver includes means responsive to a heading angle for generating acomponent of current in said coil related thereto and further includinga pendulous resistor for generating a second component of current insaid coil related to the pendulous movement thereof.
 6. A compensationsystem for a magnetic compass utilizing a Hall effect generator whereinsaid Hall effect generator comprises mutually perpendicular heading andtransverse elements, including:(a) a heading compensation coilencircling said heading element, (b) a transverse compensation coilencircling said transverse element, (c) a first compensation driver forgenerating a flow of current through said heading compensation coil tocreate a magnetic field surrounding said coil having a magnitude equalto but opposite in direction of local magnetic field components otherthan the earth's horizontal magnetic field, and (d) a secondcompensation driver for generating a flow of current through saidtransverse compensation coil to create a magnetic field surrounding saidcoil having a magnitude equal to but opposite in direction of localmagnetic field components other than the earth's horizontal magneticfield.
 7. A compensation system for a magnetic compass utilizing a Halleffect generator as set forth in claim 6 wherein said first compensationdriver includes means responsive to a heading angle for generating acomponent of current in said heading compensation coil related thereto,and wherein said second compensation driver includes means responsive toa heading angle for generating a component of current in said transversecompensation coil related thereto.
 8. A compensation system for amagnetic compass utilizing a Hall effect generator as set forth in claim7 wherein said first and second compensation drivers each includes apendulous resistor for generating a component of current in therespective compensation coils related to a pendulous movement thereof.